David Koch Steps Down From Business and Conservative Political Group

David H. Koch, the billionaire industrialist who combined his vast wealth and libertarian-tinged brand of conservatism to influence candidates and campaigns at all levels of American politics, is stepping away from his political and business interests because of declining health, his company announced Tuesday.

Mr. Koch’s brother Charles said in a letter to employees at Koch Industries, the company the two controlled, that David Koch’s health problems had made it impossible for him to continue working. “Unfortunately these issues have not been resolved and his health has continued to deteriorate,” the letter said. “We are deeply saddened by this.”

The Koch family has been influential in conservative politics for generations, long before its name became synonymous with big money in the Republican Party. David Koch, in fact, was the vice-presidential nominee for the Libertarian Party in 1980. Fred Koch, the family patriarch and father of Charles and David, who founded Koch Industries, was a founding member of the far-right, anti-Communist John Birch Society.

With millions of dollars in donations over the years, the Kochs’ contribution to today’s Republican Party has been seminal, helping to solidify doctrine that favored businesses and worked against stricter environmental regulations. Now, the absence of David Koch, 78, who was the more public and political of the brothers, will inevitably raise questions about whether the Kochs’ mission will shift further toward nonpolitical endeavors.

The family’s network had already started to scale back its campaign work and focus more of its efforts on issues like criminal justice and sentencing reform as David Koch’s health worsened and he took on a smaller portfolio.

People close to the brothers, who are two of the world’s wealthiest men, said that over the last three years David Koch became a much less frequent presence in the social and political circles he once ran in at the highest levels. Koch Industries did not disclose the nature of his illness.

The Kochs have been a driving force behind the Republican Party’s gains over the last decade, not just at the federal level but also at the state and local level where they recognized and seized an opportunity to install conservative lawmakers while Democrats were far more focused on higher office.

Using their powerful political group, Americans for Prosperity, they ramped up their political giving during the presidency of Barack Obama, whom Charles and David saw as seriously misguided and driven by a socialistic agenda that threatened the free-market, libertarian philosophy they espoused. The Obama administration’s approach to environmental and corporate regulation was, not incidentally, also a threat to their sprawling $100 billion-plus business, which involves everything from oil refining to manufacturing consumer products like Brawny paper towels and Stainmaster carpeting.

Americans for Prosperity played a pivotal role in the 2010 elections that put Republicans back into power in the House of Representatives — and catapulted the Kochs from relative obscurity to the center of the country’s political debate. The group provided much of the financial and organizational backing that helped the Tea Party movement become a national force.

Democrats accused the Kochs of using their money to back candidates who would be friendly to their pro-business, deregulatory vision, and tried to turn the brothers into the poster children for a broken and abused campaign finance system.

As the brothers’ political ambitions expanded and they created a larger coalition of Koch-affiliated groups aimed at mobilizing various constituencies — including Latinos, veterans and small-government conservatives — their network became almost a shadow political party with a sophisticated data-gathering operation and employees all over the country.

The 2016 election, however, changed their outlook and caused them to dial back their political efforts. They would have preferred almost any candidate to Donald J. Trump, whom they viewed as unserious and unwedded to any principle beyond self-advancement, and worked to promote a number of his competitors for the Republican nomination, including Senator Marco Rubio of Florida and Gov. Scott Walker of Wisconsin.

The president’s view that American trade policy needs to be more restrictive and punitive toward large partners like China is also antithetical to how the Kochs see economic policy.

Once Mr. Trump became the Republican nominee, they shifted their political giving away from the presidential campaign to focus exclusively on congressional races.

David Koch’s influence has been felt far beyond politics. His name is on theaters, museums, hospitals and cultural institutions all over New York and beyond. He has been an especially generous donor to the arts, contributing $100 million in 2008 to New York City Ballet’s Lincoln Center theater — which was then named after him — and $65 million in 2014 to the Metropolitan Museum of Art for the refurbishment of its fountains plaza.

Mr. Koch has also been a major supporter of cancer research in the United States and has donated more than $680 million to research and medical facilities in the United States since he was found to have prostate cancer in 1992. Some of his largest gifts have included a $100 million gift to the Massachusetts Institute of Technology and a $150 million donation to the Memorial Sloan Kettering Cancer Center in New York.

His gifts have not come without controversy. Scientists called for him to step down from his position on the board of the American Museum of Natural History, accusing him of promoting climate science denial. He did leave in 2016 after serving for 23 years, though his representatives said it had nothing to do with the criticism.

On Tuesday, when Charles Koch wrote to company employees to inform them of his brother’s retirement, his words read at times like a eulogy. “My thoughts of David will always be overflowing with the experiences, challenges, laughter and love of our life together,” he wrote. “David has always been a fighter and is dealing with this challenge in the same way.”

Robin Pogrebin and Liam Stack contributed reporting.

A version of this article appears in print on , on Page A17 of the New York edition with the headline: Illness Forces Koch Brother To Quit the Family Business. Order Reprints | Today’s Paper | Subscribe